This invention relates, in general, to semiconductors, and more particularly, to semiconductor devices.
High electron mobility transistors (HEMTs) are one of the most important semiconductor devices for microwave applications because HEMTs have high output power characteristics. Two of the most important factors that determine the output power of a HEMT are the drain current density and the gate-to-drain breakdown voltage of the HEMT. In particular, the output power density can be increased by increasing the drain current density while maintaining the same operating voltage. The drain current density can be increased by increasing the electron density in the channel of the HEMT.
Attempts to increase the electron density in the channel include the development of the double-side planar-doped (DSPD) aluminum gallium arsenide (AlGaAs)/indium gallium arsenide (InGaAs)/AlGaAs HEMT substrate. Higher current density HEMT devices can be obtained by increasing the doping densities in the DSPD HEMT substrate. However, the increased doping densities produce excess electrons in the AlGaAs layers, which lower the breakdown voltage and degrade other electrical characteristics of the HEMT. Other attempts to increase the sheet charge density of a HEMT include highly doping the channel layer of the HEMT. However, the high channel doping reduces electron mobility, which degrades device performance.
Accordingly, a need exists for a semiconductor device that has higher output power but does not have low breakdown voltage or low electron mobility.